The popup will weigh less and have less air drag than a hard side camper. The popup will have smaller holding tanks for fresh and gray water, and it will have a cassette toilet that needs more frequent dumping and a manual dump. I have never wanted a cassette toilet in a camper. The popup also has less inside storage space for extended travel.
For the southwest with its many miles of off the pavement trails that often have hard sloping rock to get across, I would want a popup or use a fiberglass cap with a tent (which is what I used for 15 years). For everywhere else the hard side camper is not a drawback and having twice as many days supply of drinking water and more waste tank capacity is much more important (even more so with the drought in the west).
Check out the LPG, water, waste tank, fridge size, and other aspects of the popup and standard hard side campers. The hard side will be double that of the popup. If you spend 90% of your time running trails with a full size pickup then go for the popup. If you spend less than that on the trails or would rather tow a true off road vehicle behind the truck, then go with the hard side type.
In terms of a used camper there is 20 times the available inventory of hard side campers from which to choose.